In the technology of process measurements and automation, physical parameters, such as e.g. mass flow rate, density and/or viscosity, of a medium flowing in a pipeline are often measured using inline measuring devices, which include a vibratory measurement transducer, through which the medium flows, and a measurement and operating circuit connected thereto, for effecting reaction forces in the medium, such as e.g. Coriolis forces corresponding to the mass flow rate, inertial forces corresponding to the density of the medium and/or frictional forces corresponding to the viscosity of the medium, etc., and for producing, derived from these forces, measurement signals respectively representing mass flow rate, density and viscosity.
Such measurement transducers, especially those in the form of Coriolis mass flow meters or Coriolis mass flow/density meters, are described in detail e.g. in WO-A 04/099735, WO-A 04/038341, WO-A 03/076879, WO-A 03/027616, WO-A 03/021202, WO-A 01/33174, WO-A 00/57141, WO-A 98/07009, U.S. Pat. Nos. 6,807,866, 6,711,958, 6,666,098, 6,308,580, 6,092,429, 5,796,011, 5,301,557, 4,876,898, EP-A 553 939, EP-A 1 001 254, EP-A 12 48 084, EP-A 1 448 956, or EP-A 1 421 349. For conveying the medium flowing at least at times, the measurement transducers include at least one flow tube, which is secured appropriately to a usually thicker-walled, especially tubular and/or beam-like, support cylinder or in a support frame. Additionally, the aforementioned measurement transducers have a second flow tube, which likewise vibrates, at least at times, and is mechanically coupled with the first flow tube at least via two, especially, however, four, coupling elements, also named node plates or couplers, with at least the first flow tube being constructed as a first measuring tube communicating with the pipeline and serving for conveying the medium to be measured. For producing the above-mentioned reaction forces, the two flow tubes are caused to vibrate during operation, driven by a usually electrodynamic exciter arrangement, with the two flow tubes usually executing bending oscillations, at least at times, about an imaginary oscillation axis essentially parallel to a longitudinal axis of the measurement transducer. For detecting vibrations of the flow tube, especially inlet and outlet end vibrations, and for producing at least one oscillation measurement signal representing such, such measurement transducers additionally include a sensor arrangement reacting to movements, and thus also to mechanical oscillations, of the flow tube. During operation, the above-described, internal oscillation system of the measurement transducer, formed by the at least one flow tube, the medium conveyed at least instantaneously therein, as well as at least partly by the exciter arrangement and the sensor arrangement, is excited by means of the electromechanical exciter arrangement at least at times in a wanted oscillation mode to execute mechanical oscillations at least one dominating, wanted oscillation frequency. These oscillations in the so-called wanted oscillation mode are usually, at least partly, in the form of lateral oscillations, especially when the measurement transducer is used as a Coriolis mass flow and/or density meter. Usually chosen as the wanted oscillation frequency in such cases is a natural, instantaneous, resonance frequency of the internal oscillation system, which, in turn, depends both on the size, shape and material of the flow tube and also on an instantaneous density of the medium; if necessary, the wanted oscillation frequency can also be significantly influenced by an instantaneous viscosity of the medium. Due to fluctuating density of the medium to be measured and/or due to medium changes occurring during operation, the wanted oscillation frequency during operation of the measurement transducer is naturally changeable at least within a calibrated and, thus, predetermined, wanted frequency band, which, correspondingly, has a predetermined lower, and a predetermined upper, limit frequency. The internal oscillation system of the measurement transducer formed in common by the at least one flow tube, together with the exciter and sensor arrangements, is, additionally, usually accommodated by a housing having the support frame, or support cylinder, as integral component, with the housing being mechanically coupled to the pipeline via an inlet end and an outlet end and likewise exhibiting a plurality of natural oscillation modes. Suitable transducer housings for vibratory measurement transducers are described, for example, in WO-A 03/076879, WO-A 03/021202, WO-A 01/65213, WO-A 00/57141, U.S. Pat. Nos. 6,776,052, 6,711,958, 6,044,715, 5,301,557, or EP-A 1 001 254.
Progress in the field of vibratory measurement transducers has, in the meantime, reached a level where modern measurement transducers of the described kind can be applied in practice for almost all purposes in the technology of flow measurements and can satisfy the highest requirements existing in such field. Thus, such measurement transducers can be applied to measure mass flow rates of only a few g/h (grams per hour) up to some t/h (tonnes per hour), and pressures of up to 100 bar for liquids, or even over 300 bar for gases. The accuracy of measurement achieved in such applications lies, usually, at about 99.9% of the actual value, or even above, i.e. a measurement error of about 0.1%, while a lower limit of the guaranteed measurement range can lie quite well at about 1% of the end value of the measurement range. On the basis of the high bandwidth, measurement transducers of the described kind can be offered, depending on application, also with nominal diameters, as measured at the flange, lying between 1 mm and 250 mm, or even beyond.
Investigations on vibratory measurement transducers having two, mutually parallel, curved flow tubes, such as are described e.g. in U.S. Pat. No. 6,711,958 or U.S. Pat. No. 6,308,580, have shown, however, that, despite a largely symmetrical construction with reference to an imaginary central plane of the measurement transducer extending between the two curved central tube segments of the flow tubes, alternating imbalances can be produced in significant measure in the rhythm of the wanted oscillation frequency and, consequently, associated disturbance oscillations can be coupled out into the connected pipeline. Proving to be especially harmful, in this regard, for the required, high measurement accuracy are those disturbance oscillations which act in the direction of that principal axis of inertia of the measurement transducer—in the following designated the vertical axis—which lies in the aforementioned, imaginary central axis of the measurement transducer and extends essentially perpendicular to the axis of the oscillations. To diminish such disturbance oscillations, especially those directed transversely to the oscillation axis, it is proposed both in EP-A 12 48 084 and in WO-A 04/099735 to apply a cantilever-like balance-element to a curved, central, middle tube segment of each of the two flow tubes. The effect of such balance elements lies essentially in their ability to generate acceleration forces directed counter to the acceleration forces produced by the vibrating flow tubes and directed, in the above sense, vertically to the oscillation axis, so that these forces partially cancel one another. Further investigations have, moreover, shown that, in the case of measurement transducers of the described type, especially those with V-shaped or trapezoidally bent, flow tubes and/or with flow tubes whose tube diameters amount to 80 mm or more, besides such forces coming mainly from the acceleration of moved masses, to an increasing degree, also clamping forces can also lead to significant imbalances in the measurement transducer, such as are dependent on an asymmetric deformation of the transducer housing stemming from an instantaneous deflection of the flow tubes.
FIGS. 1 and 2 are two schematic sketches for explaining the oscillatory motion in the case of a measurement transducer of the described kind having two curved, mutually parallel, flow tubes, which are mechanically coupled together at the inlet and outlet ends via, in each case, two coupling elements. The flow tubes are shown here schematically in simplified form and shown cut free at the ends, thus free of the transducer housing which otherwise holds them, so that they can, therefore, oscillate virtually at their ends. As already mentioned, the two flow tubes oscillate, during operation, relative to one another, and, indeed, in a way such that they deflect laterally (X-direction) practically over their entire lengths. The amplitudes of these deflections may differ from one another. The predominant part of the oscillations and of the associated forces is thus both perpendicular to the oscillation axis (Z-direction) and to the mentioned vertical axis (Y-direction) of the measurement transducer, wherein, at least for the case that both flow tubes are flowed through at the same time by the medium, the component of the one flow tube essentially cancels the corresponding component of the other flow tube. A smaller component of the forces caused by the oscillations acts also in the direction of the vertical axis (Y-direction). The oscillatory motion of the flow tubes is, in spite of the coupling elements, transmitted through to the —here free—ends, with also the coupling elements being slightly deformed (FIG. 2). The middles of the coupling elements move in such case also in the direction of the vertical axis, while the “free” ends of the flow tubes move oppositely in the direction of the vertical axis. This movement of the ends of the flow tubes leads in the installed and fixed state inversely to forces in the securing transducer housing, for example in the possible, connected distributor pieces, and, thus, also to deformations of the transducer housing.
A possibility for reducing such undesired forces in the mounting, which, for example, can vary a calibrated zero point of the measurement transducer, would, for example, be correspondingly to increase a stiffness of the transducer housing resisting the aforementioned deformations of the transducer housing by increasing its wall thickness. However, a special problem connected with such a measure is that, in the case of measurement transducers of large nominal diameter, the installed mass is already very high. For measurement transducers of nominal diameter far in excess of 150 mm, including flanges possibly attached thereto, the installed mass can lie easily at about 500 kg. Thus, in the case of measurement transducers of large nominal diameter, the possibilities for sufficient stiffening of the transducer housing by increased material thicknesses must be considered as very limited, at least for the desired application of proven materials, especially stainless steel.